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Author Topic: Another Gowen LPP  (Read 8719 times)
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ceandra
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« Reply #75 on: April 16, 2019, 12:04:57 AM »

OK, yes, we do that and I can see that it would be harder without that extension.

Chuck
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calgoddard
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« Reply #76 on: April 16, 2019, 09:48:52 AM »

Chuck -

Can you please give more details about your following comment from Reply #66?

"We spent some time improving our winding technique (keep it turning more at the end when coming in fast) and the bunching went away."

Thank you.
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ceandra
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« Reply #77 on: April 16, 2019, 12:10:13 PM »

Cal:

One of my kids was consistently getting a huge wad at the front end about 7 minutes into the flight, which would stop the prop and the plane would come down from 70-80 feet. Several others were getting large wads at the back, not every flight, but often, with or without sleeves. These would cause a CG shift, as well as significant unused turns.

On observing them wind, I saw that they were reaching close to their goal winds while still a foot away from the holder for the winder. If they simply stopped winding here and moved in, they would get obvious large stringers sticking out the side of the rubber, which are hard to get rid of. However, if they walked the rest of the way in with just a turn or two (10:1), they did not get stringers and all looked well, but they would get balled up in flight.

Instead, I got them to either save a few more winds for this last part, or stretch it back out a little and go over budget on winds. This made a more even distribution of medium-large knots, instead of a few larger knots. The few larger knots were not immediately apparent, but would form a nucleation site for a big mess in flight. By focusing on getting more winds in that last foot of walk-in, the bunching was more uniformly distributed, and resulted in no further instances of major bunching, with or without sleeves. Note that we generally went to 2.3g instead of 2.5 as well. But, the one that was bunching at the front also was the one that was using the lightest rubber, and her loop lengths were still at 22".

Chuck
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ceandra
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« Reply #78 on: April 16, 2019, 12:19:53 PM »

I have not yet found a video of the high power launches yet.

After thinking overnight, while these launches are certainly better than unwinding at the table, and better than racing in a circle, they are not efficient use of the energy. Ideally, the flare load would be sufficient to slow the plane down and fly more on the wing, without thrashing the air. However, then the flare must relax to the right point for efficient cruise. This probably means some combination of a softer flare, with a lower static pitch, such that the pitch goes way up on launch (but not so far as to stall the blades), and then goes to the right pitch for efficient cruise, not so low as to spin high RPM in cruise and letdown.

At the high altitude, thin air, I am not sure you can load the prop enough at 0.55 oz-in torque to slow the launch. But, we did not explore all variations of pitch and stiffness. The high power launches would last perhaps halfway to the ceiling, then it would settle into a more normal climb. Other methods explored did not get to the ceiling, and resulted in shorter flights. I would love to spend a day or two in the Dome exploring a variety of prop flare and pitch combinations! I am sure there is a more efficient setup. What we found got us very respectable times for a first time team, but in no way is the ultimate solution.

The one guy who out-flew us (perhaps his name was Avery?), my student thinks. was a guy who got it up into the opening at the center. The plane actually disappeared over the railings up there, then came back out. Not certain that was the 13:45 flight, but my student thinks it was. I did not see the climb on his, but believe it was more controlled. If you could convert those rapid turns into a controlled climb on the wing, I think you could gain a minute or more.

Chuck
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piecost
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« Reply #79 on: April 16, 2019, 06:49:36 PM »

I am enjoying this revitalised thread. Especially the comments about winding and bunching. I have certanly had terrible looking bunches on my motors, which can't be good, but have not needed to employ sleeves.

I concluded that bunches effectively prevent the rubber within them taking further turns. I believe that this causes a premature torque rise at too low winding turns. Also a lumpy torque delivery as bunches run out. Do you agree?

I like the testing of prop flair and have not quite got round to doing that myself. I have built far too many variations of pitch and number of rods on the hub to ever test. I would be better sticking with a tried and tested setup than fiddling all the time. But this is where the fun is for me. I have not even started to use washout to prevent blade stall at high torque.

I had used the inset tip ribs and binding the tips with fine fishing line before applying freshly opened cyano. I was pleased to find that steering with a roach pole against the side of the.fin did not cause the joint to fail.

I will have to check and reset the twist of my wings to prevent the warp at high torque.
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ceandra
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« Reply #80 on: April 16, 2019, 07:15:54 PM »

piecost:

The bunching I am referring to is not necessarily visible during winding, but rather manifests in flight as a big wad, either at the tail hook or at the prop hook. Even with seeming decent looking wound rubber, these occurred and caused loss of several minutes of flight. I am suspecting that even when large unsightly bunches were not visible in the wound motor, the largest knots were unevenly distributed and became bunched enough to hit the motor stick during flight. Careful attention to the last stages of winding seemed to greatly improve the situation.

Chuck
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Olbill
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« Reply #81 on: April 16, 2019, 08:15:31 PM »

I haven't seen much effect from winding technique on knot formation but I'm not saying it isn't possible. One thing that makes me dubious is how does that relate to backoff turns. It seems like whatever you did during winding would be wiped out by backoff turns.

Anyway, when I get my launch torque set and put the motor on the model I move as many knots as possible to the front. With a 1" rear sleeve, the knots moved to the front and 20" or so maximum motor length, most of my flights will have an acceptable unwinding of the motor without a cruise killing CG shift.

A6's have a 6" nose to hook maximum and use the same length motors (at least mine do)!
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ceandra
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« Reply #82 on: April 17, 2019, 12:38:23 AM »

Bill:

I do see your point. And there were many variables we were playing with. It did seem that we did better after the change in winding, but the majority switched to rubber that was 20" or under, though the one girl with the most struggles was still using 22" rubber.

So much fun learning in all of this, and I really appreciate the feedback we get on this forum.

Tonight we flew in a 20' gym, with walls right at the edge of the single court. Our planes were clearly not ideal for that gym. One student got over 5 minutes twice, and another two would have been if not for late hits on tables, chairs, and walls. We'll play with the flare stiffness before next time! The ceiling took a lot of hits.

Chuck
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